1. Field of the Invention
The present invention relates to a semiconductor light emitting device and, more particularly, to a semiconductor light emitting device using an InGaAlP system semiconductor material.
2. Description of the Related Art
Of Group III-V compound semiconductors which are lattice-matched with GaAs, an In.sub.l-y (Ga.sub.l-x Al.sub..times.).sub.y P mixed crystal (0.ltoreq.x.times..ltoreq.1 and 0.ltoreq.y.ltoreq.1) has a largest direct transition band gap and therefore has been receiving a lot of attention as a light emitting device material for a visible light region. Recently, an InGaAlP crystal layer can be formed on a GaAs substrate by a chemical vapor deposition method using an organometalic compound (to be abbreviated as an MOCVD method hereinafter), and a visible light semiconductor laser using this technique is reported.
In a light emitting diode for a visible light region, a GaAlAs system material is used in a red region to realize a device having a high luminance. Since, however, an indirect transition material such as GaP or GaAsP is used in a region having a shorter wavelength than that of the red region, no device having a high luminance equal to that of the device for the red region has been realized.
Since an InGaAlP system material has a direct transition band structure up to a green region as described above, a light-emitting diode having a high luminance throughout a wide visible light region can be realized by using this material. In order to realize such a device, however, InGaAlP having a low resistivity must be grown to decrease a system resistance of the device, but it is difficult to obtain low resistivity InGaAlP especially in a p-type layer. In order to decrease the resistivity of a p-type layer, a high-concentration p-type impurity must be doped. In the InGaAlP system material, however, if a p-type impurity is doped at a high concentration, only a part of the doped impurity can be electrically activated, i.e., activity is reduced to cause carrier concentration saturation. In addition, if an Al content is increased, a ratio of the doped impurity to be contained is reduced to limit the concentration of the doped impurity. Furthermore, mobility of carriers in InGaAlP is comparatively small. In particular, mobility of holes is as very small as 10 to 20 cm.sup.2 /V.s. Therefore, the resistivity cannot be reduced very much upon doping at a concentration of about 10.sup.18 cm.sup.- 3. For this reason, in an LED structure in which a first clad layer consisting of n-type InGaAlP, an active layer consisting of InGaAlP, and a second clad layer consisting of p-type InGaAlP are stacked in the order named on a substrate consisting of n-type GaAs holes injected from an electrode are not easily spread in the lateral direction, and most of light emission recombination in the active layer takes place below an electrode at a p-type layer side. Therefore, light emission occurs in only a peripheral portion of the p-type layer side electrode, resulting in a very low emission light extraction efficiency. Since an n-type dopant can be comparatively easily doped at a high concentration, an LED structure may be formed by stacking first clad layer consisting of p-type InGaAlP, an active layer consisting of InGaAlP, and a second clad layer consisting of InGaAlP in the order named on a substrate consisting of p-type GaAs. Mobility of electrons in InGaAlP is not so high, i.e., about 100 cm.sup.2 /V.s within a composition range having large Al content to be used in a clad layer. In order to spread an injection current in the lateral direction, therefore, the thickness of the n-type second clad layer must be increased to be several tens .mu.m or more. It is not theoretically impossible to grow a thick film having the above thickness by the MOCVD method suitable as a crystal growth method for the InGaAlP material. This technique, however, is not practical since it requires, e.g., a very long growth time or a very large amount of a Group V source gas.